Yellowstone Caldera Chronicles is a weekly column written by scientists and collaborators of the Yellowstone Volcano Observatory. This week's contribution is from Mark Stelten, research geologist with the U.S. Geological Survey.

Yellowstone caldera formed approximately 631,000 years ago due to a large (>1000 km³, or 240 mi³), eruption of rhyolite magma that deposited ash over much of the United States—one of three such eruptions during the past 2.1 million years. Over the past few decades, Yellowstone’s caldera-forming eruptions have been studied by many scientists, and much has been learned about when these eruptions occurred and their causes. For example, evidence is emerging that the events are more complex than previously appreciated, possibly including multiple smaller (but still huge) explosions spread over weeks to decades, rather than a single massive explosion.

While these catastrophic events are the most destructive type of volcanic eruption that occurs at Yellowstone, caldera-forming events are rare and represent the least likely volcanic hazard. More commonly, Yellowstone volcano erupts rhyolites as lava flows, lava domes, or in rare cases small explosive eruptions. These eruptions are referred to as intracaldera rhyolite eruptions because they take place from vents within the caldera structures.

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Following the formation of Yellowstone caldera 631,000 years ago, there have been at least 28 intracaldera rhyolite eruptions, which occurred in two stages. These lava flows and domes have filled in much of Yellowstone caldera and are one reason that you don’t see an obvious crater when you visit Yellowstone National Park. From approximately 580,000 to 250,000 years ago at least 6 rhyolites erupted within Yellowstone caldera and are known as the Upper Basin Member rhyolites. The most recent stage of intracaldera rhyolite volcanism occurred from approximately 160,000 years ago to 70,000 years ago, erupting 22 rhyolites known collectively as the Central Plateau Member rhyolites.

When driving through Yellowstone National Park, these rhyolites appear as broad plateaus or steep dome-like structures often covered with lodgepole pine trees. Individually, intracaldera rhyolite eruptions are smaller in volume (0.4 km³ to 71 km³ [0.1 mi³ to 17 mi³]) compared to those of the caldera-forming eruptions, but the cumulative volume of intracaldera rhyolites is on par with that of caldera-forming eruptions. For example, since the formation of Yellowstone caldera 631,000 years ago, more than 360 km³ (86 mi³) of rhyolite has erupted, mostly as lava flows.

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Recent work by YVO scientists published in the journal Bulletin of Volcanology set out to precisely constrain the timing of the most recent stage of intracaldera rhyolite eruptions at Yellowstone using a radiometric dating technique called ⁴⁰Ar/³⁹Ar geochronology. In this technique, scientists extract a mineral called sanidine from the rhyolite lavas and measure the ratio of potassium (the parent) to argon (the daughter product produced through radioactive decay) in the minerals, which allows the time of eruption to be precisely determined. Specifically, this work set out to test if the Central Plateau Member rhyolite eruptions occurred in clusters (where multiple eruptions occur over a short duration), if they erupted at separate and irregular time intervals, or if both conditions exist. This information is essential for understanding the frequency of rhyolite eruptions at Yellowstone and characterizing its volcanic hazards.

Results of this work show that the 22 Central Plateau Member rhyolite eruptions occurred in five brief episodes at 160,000, 150,000, 111,000, 104,000, and 71,000 years ago. During these episodes, two to nine rhyolites erupted from volcanic vents spaced out over several kilometers to tens of kilometers (a few to several miles). These episodes are estimated to have taken a maximum of 400 years but may have occurred over much shorter durations. Between 10 km³ and 130 km³ (2.5 mi³ and 31 mi³) of magma erupted during each episode (for comparison, the Mount St. Helens eruption in 1980 erupted about 0.25 km³ [0.06 mi³] of magma).

These results have two important implications. First, intracaldera eruptions are more dramatic events than previously appreciated. Instead of isolated events where a single lava flow erupts, it appears that intracaldera eruptions can involve multiple eruptions occurring in different parts of the caldera at the same time. Second, if each of the five eruption episodes is considered a single volcanic event (due to their short durations), then the Central Plateau Member rhyolites would be represented by only five volcanic events instead of twenty-two, meaning that the long-term eruption rate at Yellowstone is even lower than currently thought.

The new geochronology results show that intracaldera rhyolite eruptions are more dramatic but less frequent than previously appreciated. While they may typically lack the huge explosions that characterize caldera-forming eruptions, they can happen in groups where multiple rhyolite eruptions occur over a short duration. Don’t let Yellowstone’s smaller rhyolite eruptions fool you. They are still a big deal!